P8.5 Origin of the universe and the Big Bang theory

The Big Bang theory

The Big Bang theory states that the universe began from an extremely hot, dense state approximately 13.8 billion years ago and has been expanding ever since. It is the scientific consensus model of cosmology, supported by multiple independent lines of evidence.

"Big Bang" is a slightly misleading name — it was not an explosion in space, but rather an expansion of space itself from an initial point (singularity).

Evidence for the Big Bang

1. Red-shift of distant galaxies

All distant galaxies are moving away from us; the further the galaxy, the faster it recedes (Hubble's Law). Extrapolating backwards in time, all matter was once in one place → points to a beginning.

2. Cosmic Microwave Background (CMB) radiation

Discovered in 1965 by Penzias and Wilson (they initially thought it was pigeon droppings on their antenna!). The CMB is:

• Uniform microwave radiation from every direction in the sky.
• At a temperature of about 2.7 K.
• The cooled remnant of the radiation that filled the early universe (the universe was opaque plasma for the first ~380 000 years; when it cooled enough for atoms to form, photons were released — we now detect these as CMB).

3. Abundance of hydrogen and helium

Immediately after the Big Bang, nuclear fusion produced mostly hydrogen (≈ 75 %) and helium (≈ 25 %), with tiny amounts of lithium. The observed ratio matches Big Bang nucleosynthesis predictions exactly.

Unanswered questions

• Dark matter: extra gravity observed in galaxies and galaxy clusters that cannot be explained by visible matter. Thought to be an unseen form of matter.
• Dark energy: the expansion of the universe is accelerating, not slowing. A mysterious energy permeating space seems to be driving this. Together, dark matter and dark energy make up ≈ 95 % of the content of the universe.

Exam focus

• Know two pieces of evidence: red-shift and CMB.
• CMB: what it is, where it comes from, and what temperature it corresponds to.
• Dark matter and dark energy are mentioned as "open questions" — you don't need equations for these.